Superconducting magnet

ABSTRACT

This invention provides a superconducting magnet apparatus capable of preventing or restraining the heat invading into the apparatus and reducing the refrigeration load of an external refrigerator, at the time of changeover of a switch in transiting to a persistent current mode, and capable of quick changeover operation. In the superconducting magnet apparatus of the present invention ( 1 ), in transiting to a persistent current mode, a thermal superconducting switch ( 41 ) placed in a low-temperature domain and a mechanical switch ( 42 ) placed in a mid-temperature domain are turned on. Since the mechanical switch ( 42 ) is placed in the mid-temperature domain, even if a heat load is generated through the drive mechanism ( 60 ), it is not necessary to cool the mechanical switch to an extremely low temperature. In short, heat generated at the contact point of the mechanical switch ( 42 ) and heat invasion through the drive mechanism ( 60 ) are applied not to the low-temperature domain, but to the mid-temperature domain, so that an external refrigerator can easily absorb the heat load and the capacity of the refrigerator can be reduced.

TECHNICAL FIELD

This invention relates to a superconducting magnet apparatus thatswitches itself into a persistent current mode by changeover of switchesafter excitation.

BACKGROUND ART

Along with improvements in performance of the superconducting wires andadvances in coil manufacturing techniques using such wires, as well astechnical developments in related apparatuses such as heat-insulatingcontainers and refrigerators, various types of superconducting magnetsand application apparatuses employing such magnets have been created.Among these, there is a type which is operated in a persistent currentmode. Superconducting magnet apparatuses for magnetic resonance imagingsystems (MRI) and for magnetically levitated vehicles (Maglev) areexamples of this type which have already been put into practical use.These superconducting magnet apparatuses supply an electric current froman external excitation power source to a coil that is cooled to anextremely low temperature. While a required magnetic field is produced,winding start and end portions of the coil are shunted by asuperconducting switch, and this makes the apparatus run in a persistentcurrent mode in which the electric current continues to flow into thecoil without a power supply. FIG. 5 shows an example of an excitationcircuit in these conventional superconducting magnet apparatuses.

As shown in FIG. 5, superconducting switch 140 is connected to each endof the winding start portion and the winding end portion of asuperconducting coil 110 in a conventional superconducting magnetapparatus 101 and the superconducting coil 110 is placed in an extremelylow-temperature area (about 4.2K) inside the superconducting magnetapparatus 101. A normal conducting current lead 132 having a low thermalconductivity is also connected to each end of this superconducting coil110. The other end of this normal conducting current lead 132 extendingto the outer surface of the superconducting magnet apparatus 101 isconnected to an external excitation power source 151 in anormal-temperature domain (about 300K).

Conventionally, a thermal superconducting switch, whose resistance iszero when it is on and which has a simple structure, has been mainlyused as an aforementioned superconducting switch 140. However, amechanical superconducting switch, and a superconducting switchcomprising a thermal superconducting switch 141 and a mechanicalsuperconducting switch 142 connected in parallel as shown in FIG. 5,have been proposed (see patent document 1 and non-patent document 1, forexample).

[Patent Document 1]

-   Unexamined Japanese Patent Publication No. 6-350148    [Non-Patent Document 1]-   “Handbook of Research and Development of Superconductivity”,    International Superconductivity Technology Center, published by    Ohmsha, Ltd., P 160-163, (1991)

However, conventional superconducting magnet apparatuses have thefollowing problems, when each superconducting switch is adopted in theexcitation circuit.

In the case of using a thermal superconducting switch alone, there is adisadvantage that it takes time to cool or to heat between ON-state(superconducting state) and OFF-state (normal conduction state). Inother words, it takes time to change from ON-state to OFF-state and viceversa, since it utilizes thermal phenomena. Especially in the case ofusing superconducting coils having a high superconducting criticaltemperature and also employing a material having a high superconductingcritical temperature for a superconducting switch in order to keep thesuperconducting state in the persistent current mode, it takes a longertime to change from the ON-state to the OFF-state and vice versa, sincethe temperature difference between ON-state and OFF-state is larger andthe thermal capacity is larger. As a consequence, during excitation whenthe superconducting magnet apparatus is switched into the persistentcurrent mode, the energized time of the current lead becomes longer andthe Joule heat increases, thus causing a problem of an increase in theload of an external refrigerator which cools inside of thesuperconducting magnet apparatus.

In the case of employing a mechanical superconducting switch alone, itis possible to turn it on and off instantly, but it is difficult todecrease contact resistance sufficiently in an ON-state, so the contactresistance causes problems such as current decay and heat generationwhen the switch is connected to the superconducting coil. Furthermore,there is a disadvantage in that considerable invasion heat from thedrive mechanism that drives the contact in a contact or non-contactstate increases heat load of the superconducting coil.

As shown in FIG. 5, in the case of connecting a thermal superconductingswitch and a mechanical superconducting switch in parallel, as in thecase of employing a mechanical superconducting switch alone, there isthe same disadvantage that invasion heat from the drive mechanism thatdrives the contact increases the heat load of the superconducting coil.There also is a problem that the contact resistance of the mechanicalsuperconducting switch causes heat generation until the thermalsuperconducting switch has completed switching when turned on.

One object of the present invention, which was made in view of the aboveproblems, is to provide a superconducting magnet apparatus capable ofpreventing or controlling heat invasion into the inside of the apparatusat the time of changeover of the switching, thereby reducing the coolingload of the external refrigerator, and capable of quick changeoveroperation.

DISCLOSURE OF THE INVENTION

In order to reach the above object, a superconducting magnet apparatusset forth in claim 1 is comprised of a vacuum container including aninner tank forming therein a low-temperature domain and housing asuperconducting coil, a heat shield covering the inner tank and formingtherein a mid-temperature domain, with a temperature higher than in thelow-temperature domain, and an outer tank containing the heat shield soas to separate the heat shield from the ambient air and forming anormal-temperature domain with a temperature higher than in themid-temperature domain; a current lead connected at one end to thesuperconducting coil, and connected at the other end to a lead lineleading to an external excitation power source provided outside thevacuum container; a switch turned on and off in order to shunt or openboth ends of the superconducting coil; and a switching device that turnson and off the switch.

The “low-temperature domain” means a so-called extremely low-temperaturedomain with almost the same temperature as the cooling temperature ofthe superconducting coil; the “normal-temperature domain” means atemperature domain with almost the same temperature as the roomtemperature; and “mid-temperature domain” means a temperature domainwith a temperature higher than the temperature in the low-temperaturedomain and lower than the critical temperature of the superconductingcurrent lead.

“Thermal shield” means a shield which is called an irradiation shield aradiation shield or a thermal shield.

The “low-temperature domain”, as mentioned above, is the domain which iscooled to an extremely low temperature, and there are several coolingsystems, such as, one in which a cooling medium like liquid helium issupplied to the inside of the inner tank from an external refrigerator,and another in which cooling is created by heat conduction from theexternal refrigerator. In a similar fashion, as regards “mid-temperaturedomain”, there are several cooling systems such as one for cooling bysupplying a cooling medium like liquid nitrogen from the externalrefrigerator and another for cooling by heat conduction from theexternal refrigerator.

The superconducting apparatus is switched to a persistent current modeby supplying an electric current to the superconducting coil from theexternal excitation power source through the current lead under acondition that the switch is off, thereby causing the superconductingcoil to generate a necessary magnetic field, and then shunting both endsof the superconducting coil by turning on the switch.

Specifically, the current lead includes a first current lead connectedat one end to one end of the superconducting coil in the low-temperaturedomain and a second current lead connected at one end to the other endof the first current lead in the mid-temperature domain and connected atthe other end to the lead line in the normal-temperature domain, atleast the first current lead including the superconducting current leadwhich becomes a superconducting state in the mid-temperature domain.

Also, the switch includes a thermal superconducting switch connected tothe both ends of the superconducting coil to shunt the superconductingcoil in the low-temperature domain and a mechanical switch connected tothe both ends of the first current lead in parallel to the thermalsuperconducting switch and able to shunt the superconducting coil in themid-temperature domain. In other words, while the thermal switch (thesuperconducting switch) disposed in the low-temperature domain includesa superconducting switch which is switched to a persistent current mode,the mechanical switch disposed in the mid-temperature domain may beeither a superconducting switch or a normal-conducting switch.

The mechanical switch is turned on and off through a driving mechanismdisposed in the mid-temperature domain and driven by the switchingdevice.

The term “thermal superconducting switch” means a switch that is turnedoff by electrical resistance caused by transition to the normalconducting state when it is heated by a heating device, such as aheater; and “mechanical switch” means a switch which is turned on by amechanical structure.

According to the above configuration, when the switch is turned on toswitch into a persistent current mode in an excited state, it takes acertain amount of time to complete the switching process since thethermal superconducting switch uses the thermal phenomena, but theoutput current of the external power source can be attenuated (shut off)immediately by the instant switching of the mechanical switch. As aconsequence, since the current-carrying time of the current lead isshortened and the heat generation is reduced, the load of therefrigerator that cools the current lead can be reduced.

In the case of using the above configuration, since the mechanicalswitch is placed in the mid-temperature domain, even if a heat load isgenerated through the drive mechanism, it is not necessary to cool themechanical switch to an extremely low temperature. In short, heatgenerated at the contact point of the mechanical switch and heatinvasion through the drive mechanism are applied not to thelow-temperature domain, but to the mid-temperature domain, so that theexternal refrigerator can easily absorb the heat load.

As a consequence, the capacity of the refrigerator can be reducedcompared with the conventional case of placing a mechanical switch inthe low-temperature domain.

When the aforementioned second current lead is a normal conductingcurrent lead, which is made of a normal conducting material, thesuperconducting magnet apparatus can be produced at a lower cost thanwhen the second current lead is made of a superconducting material.However, a part or all of the second current lead may be asuperconducting current lead that is made of a superconducting material.

Additionally, a mechanical switch does not necessarily need to be madeof a superconducting material, since heat load through the mechanicalswitch is allowed to some extent.

Therefore, as set forth in claim 2, if a normal conducting switch, whichis made of a normal conducting material, constitutes the abovemechanical switch, a superconducting magnetic apparatus can be producedat a lower cost.

In addition, as mentioned above, even if a normal conducting materialthat is less expensive than a superconducting material constitutes thesecond current lead and the mechanical switch, the load to the externalrefrigerator cannot be reduced while maintaining the inherent functionsas a superconducting magnet apparatus without applying the configurationmentioned in claim 1, which effectively utilizes the mid-temperaturedomain.

After the thermal superconducting switch reaches the givensuperconducting state and transition to a persistent current mode iscompleted, the mechanical switch may be kept on by the drive mechanismas well as the thermal superconducting switch.

In the above configuration, if some sort of thermal agitation is appliedto the thermal superconducting switch in the persistent current mode tocause temporary transfer to the normal conducting state, thesuperconducting coil can keep the excitation state since the currentmakes a detour thorough the mechanical switch. In short, the mechanicalswitch will not be turned off by thermal agitation, while the thermalsuperconducting switch returns to the superconducting state when thethermal agitation is removed. Therefore, stabilized current-carrying inorder to return to the persistent current mode can be achieved.

In the situation in which the current-carrying of the thermalsuperconducting switch can be fully stabilized in relation to thethermal agitation, and transition to a normal conducting state isimpossible, as set forth in claim 3, the aforementioned switching deviceturns on the mechanical switch and the thermal superconducting switch atthe start of switching into a persistent current mode and turns off themechanical switch when the thermal superconducting switch reaches thegiven superconducting state.

By the above configuration, in the case in which the drive mechanismcontinues applying the driving force in order to keep the mechanicalswitch on and the drive mechanism generates power loss, in other words,heat, the drive mechanism stops generating heat by turning off themechanical switch and the load of the external refrigerator that coolsthe mid-temperature domain where those switch and mechanism are placedcan be reduced more.

As set forth in claim 4, the above drive mechanism may be a mechanismusing a normal conducting solenoid that generates electromagnetic forceby the interaction with the magnetic field which the superconductingcoil generates, and turning on and off the mechanical switch by closingor opening of the contact of the mechanical switch by theelectromagnetic force. An example of this mechanism is shown as anembodiment hereinafter described.

According to the above constitution, by using effectively the magneticfield that superconducting coil generates and by a simple constitution,the mechanical switch can be switched.

Alternatively, as set forth in claim 5, the drive mechanism may be amechanism using a piezoelectric element, and turning on and off themechanical switch by closing or opening the contact of the mechanicalswitch by energization or deenergization of the piezoelectric element.

Specifically, the drive mechanism may be designed to move back and forthby an extension/contraction action of the piezoelectric element, such asa piezoelectric ceramic, which extends in response to a voltage appliedand is made to extend/contract by energization control, and thereby toclose or open the mechanical switch.

The above piezoelectric element is less influenced by a magnetic field,so the magnetic field distribution in the superconducting magneticapparatus doesn't restrict the arrangement of the piezoelectric element.Also, even when the magnetic field is changed by, for example, changingthe magnetomotive force of the superconducting coil, the mechanicalswitch may be driven reliably.

Alternatively, as set forth in claim 6, the drive mechanism may be aslide mechanism including an ultrasonic motor using a piezoelectricceramic.

When the power supply is stopped, the above ultrasonic motor stops,while maintaining the state at the point. Thus, when the power supply isstopped while the mechanical switch is on, the mechanical switch is keptin an ON-state. When the mechanical switch should be kept in an ON-statein order to prevent the thermal superconducting switch from transitingto a normal conducting state in accordance with the above describedconstitution, this provides a power-saving feature since power supplydoesn't need to be continued in order to keep the ON-state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view showing a schematic constitution of asuperconducting magnet apparatus according to a first embodiment of thepresent invention;

FIG. 2 is an explanatory view showing an electric constitution of asuperconducting magnet apparatus according to the first embodiment;

FIG. 3 is an explanatory view showing a schematic constitution of adrive mechanism which composes the superconducting magnet apparatus ofthe first embodiment;

FIG. 4 is an explanatory view showing a schematic constitution of adrive mechanism which composes a superconducting magnet apparatus of asecond embodiment; and

FIG. 5 is an explanatory view showing an electric constitution of aconventional superconducting magnet apparatus and its problems.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will now be described forfurther clarification of the mode for carrying out the invention, withreference to the accompanying drawings.

First Embodiment

The present embodiment describes a superconducting magnet apparatus forMaglev according to the present invention. FIG. 1 is an explanatory view(cross sectional view) showing a schematic constitution of thesuperconducting magnet apparatus. FIG. 2 is an explanatory view showinga schematic electric constitution of the superconducting magnetapparatus.

As shown in FIG. 1, the superconducting magnet apparatus 1 of thepresent embodiment comprises a vacuum container 20 housing asuperconducting coil 10 inside the vacuum container 20, and a currentlead 30 placed in the vacuum container 20 for supplying an electriccurrent to the superconducting coil 10 from an external excitation powersource 51.

The vacuum container 20 comprises an inner tank 21 in which alow-temperature domain is formed and the superconducting coil 10 isplaced, a heat shield 22 covering the inner tank 21 in which amid-temperature domain whose temperature is higher than thelow-temperature domain is formed, and an outer tank 23 in which the heatshield 22 is contained so as to be separated from the ambient air and ahigh-temperature domain is formed, whose temperature is higher than themid-temperature domain.

In this embodiment, since an NbTi superconducting alloy wire is employedas the superconducting coil 10, the above “low-temperature domain” isset to about 5K or below. In the case that a superconducting wire whosetemperature dependence of critical current is different from NbTi, forexample, Nb₃Sn superconducting wire is employed, the above“low-temperature domain” may be set to about 15K or below and in thecase that a Bi-system superconducting wire is applied, it may be set toabout 60K or below. Moreover, as described later, if this embodimentemploys a high-temperature superconducting current lead of a Y-systemsuperconducting wire whose critical temperature is about 90K as part ofthe current lead 30, the “mid-temperature domain” is set to about 80K orbelow. The “normal-temperature domain” is a temperature domain with atemperature of about 300K which is the same as the temperature of theouter tank, in other words the room temperature.

The inner tank 21 is filled with liquid helium in order to cool thesuperconducting coil 10 at an extremely low temperature (about 4.2K) andthe liquid helium can be accordingly supplied from an externalrefrigerator (not shown). Liquid nitrogen supplied from the externalrefrigerator cools the heat shield 22 to a mid-temperature (about 80K).Therefore, that can reduce radiation heat (heat invasion) from the outertank 23, which forms the normal-temperature domain (about 300K), to theinner tank 21, which forms the low-temperature domain.

The current lead 30 is connected to the superconducting coil 10 at oneend, and at the other end it is connected to a lead line 55 leading tothe external excitation power source 51 provided outside the vacuumcontainer 20. This current lead 30 is composed of a first current lead31, which is a high temperature superconducting current lead consistingof the aforementioned Y-system superconducting wire which becomessuperconducting at the mid-temperature domain, and a second current lead32, which is a normal conducting current lead consisting of normalconducting metal such as copper and brass.

The first current lead 31 is connected at one end to one end of thesuperconducting coil 10 in the low-temperature domain. On the otherhand, the second current lead 32 is connected at one end to the otherend of the first current lead 31 in the mid-temperature domain andconnected at the other end to the lead line 55 outside of the vacuumcontainer. Therefore, the first current lead 31 has a thermal gradientfrom an approximate extremely low temperature (about 4.2K) near theinner tank 21 and an approximate mid-temperature (about 80K) near theheat shield 22. Transition to a superconducting state is reached at atemperature of about 80K or below. In addition, the second current lead32 has a thermal gradient from an approximate mid-temperature (about80K) near the heat shield 22 to an approximate normal temperature (about300K) near the outer tank 23.

A thermal superconducting switch 41, which is connected to both ends ofthe superconducting coil 10 and shunts it, is disposed in thelow-temperature domain, and a mechanical switch 42, which is connectedto both ends of the first current lead 31 in parallel to the thermalsuperconducting switch 41 and is able to shunt the superconducting coil10, is disposed in the mid-temperature domain.

When heating means, such as a heater, stops heating the thermalsuperconducting switch 41, the thermal superconducting switch 41 iscooled, and transits to the superconducting state with a zeroresistance, and thus is completely turned on. On the other hand, a drivemechanism 60 disposed in the mid-temperature domain turns the mechanicalswitch 42 on and off. A not-shown control unit (switch means) disposedoutside of the superconducting magnetic apparatus 1 controls the currentto the heating means, such as the aforementioned heater, and controlsthe driving current supply to the driving mechanism 60.

FIG. 3 shows a schematic configuration of the driving mechanism 60. Thedriving mechanism 60 is comprised of a short-circuit member 61 disposedalong a guide member 71 in a base portion extending from a casing (notshown) of the vacuum container 20 and disposed so that it can be movedback and forth in the approaching or separating direction to or from thefirst current lead 31. The short-circuit member 61 is comprised of abody member 62, which slides guided by the guide member 71, an elongatedsupport member 63 disposed to extend from the middle of the surface ofthe body member 62 on the side of the first current lead 31 toward thefirst current lead 31, and a plate-like contact member 64 connected tothe end of the support member 63. A normal conducting solenoid 65 isdisposed inside of the body member 62 and an end of a coil spring 67 isconnected to the outside edge of the contact member 64 through aconnection member 66. The coil spring 67 with the other end fixed to thebase member biases the contact member 64 in a direction opposite to thefirst current lead 31. On the other hand, respective contact buttons 31a, 31 a, extend from respective ends of a pair of the first currentleads 31 toward the side of the driving mechanism 60.

Therefore, when the superconducting coil generates a magnetic field dueto excitation, and the external drive power source 52 supplies currentto the normal conducting solenoid 65, the normal conducting solenoid 65generates a magnetic force by interaction with the magnetic fieldgenerated by the superconducting coil 10. This magnetic force moves thebody member 62 towards the first current lead 31 against the biasingforce of the coil spring 67. After that, the contact member 64 contactsboth of the contact buttons 31 a, 31 a and accordingly shunts thesuperconducting coil 10. In this case, when the external drive powersource 52 stops supplying current, the aforementioned magnetic forceextinguishes and the body member 62 moves back, and then thesuperconducting coil 10 is freed from the short-circuited state.

According to the superconducting magnet apparatus of the presentembodiment explained above, in transiting to the persistent currentmode, the thermal superconducting switch 41 and the mechanical switch 42are turned off, and then the external excitation power source 51supplies current to the superconducting coil 10 through the secondcurrent lead 32 and the first current lead 31. After a required magneticfield is generated around the superconducting coil 10, these switchesare turned on, and the both ends of the superconducting coil 10 areshort circuited.

In this case, while the thermal superconducting switch 41 utilizingthermal phenomena requires a certain amount of time to completeswitching, the mechanical switch can be turned on and off at once, thenthe output current of the external excitation power source 51 can beeither cut off or decayed. This results in a decrease in theenergization time for both current leads. Thus, heat generation in theleads decreases, and then the load of the external refrigerator whichcools them decreases.

If the thermal superconducting switch 41 temporarily transits to thenormal conducting state in the persistent current mode because ofthermal agitation, the superconducting coil 10 can keep the excitationstate while the mechanical switch 42 is on and the current bypasses tothe mechanical switch 42. In other words, since the mechanical switch 42is not turned off due to thermal agitation, and the thermalsuperconducting switch 41 returns to the superconducting state when thethermal agitation is removed, stabilized energization can be achieved toreturn to the persistent current mode.

In the superconducting magnet apparatus of the present embodiment, sincethe mechanical switch 42 is disposed in the mid-temperature domain, evenif the heat load is generated through the driving mechanism 60, it isunnecessary to cool the mechanical switch 42 to an extremely lowtemperature. In other words, heat generation at the contact of themechanical switch 42 and heat invasion from the driving mechanism 60 areloaded not into the low-temperature domain but into the mid-temperaturedomain, and the external refrigerator can easily absorb the heat load.Therefore, the refrigerator capacity is less than that in the case ofdisposing the mechanical switch 42 in the low-temperature domain in aconventional manner.

In addition, since the superconducting coil 10 and the mechanical switch42 are connected through the first current lead 31, which transits tothe superconducting state at mid-temperature, if the mechanical switch42 is turned on during switching into the persistent current mode, decayof the current from the superconducting coil 10 can be prevented or berestrained, and the switching into the persistent current mode can becompleted smoothly.

Furthermore, in this embodiment, in which the mechanical switch 42 isdisposed in the mid-temperature domain, the heat load can be permittedmore than in the case of disposing it in the low-temperature domain, asmentioned above. Therefore, as mentioned above, the mechanical switch 42can consist of inexpensive normal conducting material. Also, the secondcurrent lead 32 can consist of inexpensive normal conducting material.

As a consequence, the superconducting magnetic apparatus can be producedat a cheaper price, while keeping its primary performance.

As well, in this embodiment, the first current lead 31 is ahigh-temperature superconducting current lead made of a Y-systemsuperconductor, but the Y-system superconductor may be replaced by ameans which switches into the superconducting state at a temperaturelower than the mid-temperature. Thus, for example, a Bi-systemhigh-temperature superconducting current lead or T1-systemhigh-temperature superconducting current lead may be employed.

Second Embodiment

In the above first embodiment, the driving mechanism 60 utilizing thenormal conducting solenoid 65 is employed as a driving mechanism to turnon and off the mechanical switch 42. The present embodiment employs adriving mechanism that is different from the first embodiment. FIG. 4 isa schematic view of the driving mechanism, which corresponds to FIG. 3in the first embodiment. The basic constitution of the presentsuperconducting magnet apparatus, the manner of supplying electricpower, etc. are substantially identical in principle with those in thefirst embodiment.

Therefore, the identical components are numbered the same and anexplanation thereof is not repeated.

As shown in FIG. 4, a driving mechanism 80 in this embodiment comprisesa slide mechanism 81 that performs sliding movement by applying a givenvoltage to an ultrasonic motor from the external drive power source 52to drive the same.

The slide mechanism 81 comprises an ultrasonic motor 82 includingpiezoelectric ceramic and fixed on a base member 72, which is providedon the casing (not shown) of the vacuum container 20, a ball screw 83connected to a rotation axis of the ultrasonic motor 82 and extending inthe axial direction, and a short-circuit member 91 driven by rotation ofthe ball screw 83 and moving back and forth in the direction of thefirst current lead 31.

The short-circuit member 91 comprises a body member 92, which slidesguided by a guide member 73 on the base member 72, a longitudinalsupport member 93 disposed to extend from the middle of the surface ofthe body member 92 on the side of the first current lead 31, and aplate-like contact member 94 connected to the end of the support member93. A screw hole 92 a is provided, along the axis of the ball screw 83in the body member 92. A female screw, which engages the screw thread ofthe ball screw 83, is formed in the screw hole 92 a.

In switching into the persistent current mode, a voltage is firstsupplied to the drive mechanism 80 from the external drive power source52, and the ultrasonic motor 82 is driven to rotate the ball screw 83.As a result, the body member 92 slides, guided by the guide member 73,and moves toward the first current lead 31, and then the contact member94 comes into contact with contact buttons 31 a, 31 a and finally shuntsthe superconducting coil 10.

After the switching to the persistent current mode is completed, thecontact member 94 may be retreated to release the superconducting coil10 from the shunt state by starting the voltage supply from the externaldrive power source 52 and driving the ultrasonic motor 82 in reverse tothe above. Alternatively, after the switching to the persistent currentmode is completed, the superconducting coil 10 may be kept in theshort-circuit state by stopping the voltage supply from the externaldrive power source 52.

As mentioned above, when the power supply is stopped, the ultrasonicmotor 82 stops maintaining the state at the point. On this account, whenthe power supply is stopped while keeping the mechanical switch 42 on,the ON-state may be maintained. This is advantageous, in view ofpower-saving, to the above described configuration, in which themechanical switch is kept in the ON-state in order to prevent thethermal superconducting switch 41 from transiting to the normalconducting state, since it is unnecessary to keep supplying current inorder to keep the ON-state.

Moreover, since the ultrasonic motor 82 comprising a piezoelectricceramic (piezoelectric element) is employed therein, the drivingmechanism 80 is less affected by the magnetic field, so that themagnetic field distribution in the superconducting magnetic apparatuswill not restrict the arrangement of the driving mechanism 80. Even ifthe magnetic field changes, for example, by changing the magnetomotiveforce of the superconducting coil 10, the mechanical switch 42 cancertainly be driven.

The embodiments of the present invention have been described in theabove. However, embodiments of the present invention should not belimited to the above embodiments, and other variations might be possiblewithout departing from the technical scope of the invention.

In the above second embodiment, the ultrasonic motor, comprising apiezoelectric ceramic, is described as the driving mechanism comprisinga piezoelectric element. The piezoelectric element, such aspiezoelectric ceramic, extends when voltage is applied. The contact ofthe mechanical switch 42 may be designed to be closed or opened bymoving the driving mechanism back and forth, utilizingextension/contraction of the piezoelectric element by energizationcontrol. As the piezoelectric element, piezoelectric single crystal andpiezoelectric organic matter may be employed, other than thepiezoelectric element.

In the above embodiments, for instance, the second current lead 32 andthe mechanical switch 42 are composed of an inexpensive normalconducting material. However, these may surely be composed of asuperconducting material.

Moreover, in the above embodiments, as a cooling system for thelow-temperature domain in the inner tank 21, the external refrigeratorsupplies a cooling medium, such as liquid helium, into the inner tank inorder to cool the low-temperature domain. However, it may be cooled bythermal conduction from the external refrigerator. In a similar way, theexternal refrigerator supplies a cooling medium, such as liquidnitrogen, as a cooling system for the mid-temperature domain of the heatshield 22. However, it may be cooled by thermal conduction from theexternal refrigerator.

INDUSTRIAL AVAILABILITY

According to this invention, it is possible to provide a superconductingmagnet apparatus, for example, a superconducting magnet apparatus forMaglev, capable of preventing or restraining the amount of the heatinvading into the apparatus and reducing the refrigeration load of anexternal refrigerator, at the time of changeover of a switch, andcapable of quick changeover operation.

1. A superconducting magnet apparatus comprising: a vacuum containerincluding an inner tank forming therein a low-temperature domain andhousing a superconducting coil, a heat shield covering the inner tankand forming therein a mid-temperature domain with a temperature higherthan in the low-temperature, domain, and an outer tank containing theheat shield so as to separate the heat shield from the ambient air andforming a normal-temperature domain with a temperature higher than inthe mid-temperature domain; a current lead connected at one end to thesuperconducting coil, and connected at the other end to a lead lineleading to an external excitation power source provided outside thevacuum container; a switch to be turned on and off in order to shunt oropen both ends of the superconducting coil; and a switching device thatturns on and off the switch, the superconducting apparatus beingswitched to a persistent current mode by supplying an electric currentto the superconducting coil from the external excitation power sourcethrough the current lead under a condition that the switch is off,thereby causing the superconducting coil to generate a necessarymagnetic field, arid then shunting both ends of the superconducting coilby turning on the switch, wherein the current lead includes a firstcurrent lead connected at one end to one end of the superconducting coilin the low-temperature domain and a second current lead connected at oneend to the other end of the first current lead in the mid-temperaturedomain and connected at the other end to the lead line in thenormal-temperature domain, at least the first current lead including thesuperconducting current lead which becomes a superconducting state inthe mid-temperature domain, and wherein the switch includes a thermalsuperconducting switch connected to the both ends of the superconductingcoil to shunt the superconducting coil in the low-temperature domain anda mechanical switch connected to the both ends of the first current leadin parallel to the thermal superconducting switch and able to shunt thesuperconducting coil in the mid-temperature domain, the mechanicalswitch being turned on and off through a driving mechanism disposed inthe mid-temperature domain and driven by the switching device.
 2. Thesuperconducting magnet apparatus as set forth in claim 1, wherein themechanical switch is a normal conducting switch composed of a normalconducting material.
 3. The superconducting magnet apparatus as setforth in claim 1 or claim 2, wherein the switching device turns on themechanical switch at the start of switching into the persistent currentmode and turns off the mechanical switch when the thermalsuperconducting switch becomes a specified superconducting state.
 4. Thesuperconducting magnet apparatus as set forth in one of claims 1-3,wherein the driving mechanism includes a mechanism using a normalconducting solenoid generating electromagnetic force by the interactionwith the magnetic field generated by the superconducting coil, and turnson and off the mechanical switch by closing or opening the contact ofthe mechanical switch by the electromagnetic force.
 5. Thesuperconducting magnet apparatus as set forth in one of claims 1-3,wherein the driving mechanism includes a mechanism using a piezoelectricelement, and turns on and off the mechanical switch by closing oropening the contact of the mechanical switch by energizing orde-energizing the piezoelectric element.
 6. The superconducting magnetapparatus as set forth in claim 5, wherein the driving mechanism is aslide mechanism using an ultrasonic motor including the piezoelectricelement.